You have been talking about AT's reactions in scenes where she's not the main character, and I agree with what everyone has said. She is always "on", even when she's not the focus of the scene. I was watching "Urgo" today, and loved the scene where Sam is talking to Urgo (he's invisible here) whilst she's walking down the corridor. General Hammond and Janet appear and there's a funny conversation between them all. Janet and General Hammond then continue to discuss the problem and are the focus of the scene, but you see Sam in the background continuing talking with Urgo. I think Amanda's acting here is great.
I'm sorry the pics are not great, but perhaps our Royal Photographer can do a better version of this.